new shed on concrete slab

AliceW

Section62 said:

AliceW said:

I've recently had an old brick garage (20ft X 10ft) demolished and plan to have a 16ft X 8ft pressure treated wooden shed with floor bearers installed on the concrete slab -- the slab will be bigger than the shed. Pent style shed with a gutter along the short side next to the fence.  I've noticed that after heavy rain the slab has puddles. Does this mean that water is likely to get under the new shed and rot the timber bearers?

Ideally shed bases should be slightly smaller than the shed to eliminate that problem, but as you have the base already all you can reasonably do is to minimise the risk.  How much of a problem you might get depends on which way the base falls, the direction of the bearers and the amount of treatment the bearers are given.  'Pressure treated' wood in contact with the ground won't necessarily last that long anyway.

If it were me I'd put the shed where the red line is on this picture -

The reason for that is you'll have some overhang on two of the most exposed sides, so they won't be a problem.  Having a ~2 foot gap along the back next to the fence will help with maintenance, but with a 4" gutter (don't use the 3" 'mini' type as they are nothing but trouble) the 'open' gap to the fence will be ~20".  This means the rain would need to be near-vertical before it gets to the concrete base along that edge.

The downside of that position - if the door is in the near short side - is having a puddle immediately in front of the door.  But if that is an issue you could put some slabs or decking boards down on the concrete to keep your feet dry.

Are you putting the shed up yourself?  Again, if it were me I'd give the bearers a couple of coats of something like 'creocote'.  It isn't as good as traditional creosote, but is much better at protecting damp wood long-term than typical pressure treatment chemicals.  If the shed is being put up for you by the suppliers then they probably won't agree to use creocote, and will claim their pressure treatment is all you need.

Raising the bearers off the ground by using bricks or slabs would be another option - but you need to use a lot and place them very carefully, otherwise the bearers won't be supported enough and the floor will 'bounce' and sag.

Thank you. I like the idea of positioning it where  the red lines have been drawn on my  picture.The edges of the slab have a 5" cement border all the way round that slopes slightly away from the slab, but the shed would probably be positioned over that rather than leaving it exposed?

I haven't got anyone in the family who can put the shed up  so I'm arranging for a local shed manufacturer to install it and another firm to put up a gutter and water butts (as the firm putting it up can't do gutters.They do have an excellent reputation though, which is why I'm using them.) 

 

From an appearance point of view, if I have the shed where the red lines have been drawn, maybe I should think about having a longer shed -- I thought a longer  one might be more prone to leaking and more difficult to maintain? and I only need it for storing garden equipment and garden toys. A longer one would probably look better though.18ft instead of 16ft.Or maybe even 20ft.

AliceW

EssexExile said:

My wooden shed has been sitting on concrete for over 20 years with no problems.

As some have said it's a problem that the shed is smaller than the base, would it be possible to have the shed made to fully cover the base? A shed is never too big!

Thank you. I think I might go for a bigger one.

Section62

AliceW said:

Thank you. I like the idea of positioning it where  the red lines have been drawn on my  picture.The edges of the slab have a 5" cement border all the way round that slopes slightly away from the slab, but the shed would probably be positioned over that rather than leaving it exposed?

Yes, I'd take the shed right up to the edge.  Ideally putting a piece of weatherboard over the ends of the bearers and down the edge of the slab to help keep stuff out from underneath the shed.

AliceW said:

I haven't got anyone in the family who can put the shed up  so I'm arranging for a local shed manufacturer to install it and another firm to put up a gutter and water butts (as the firm putting it up can't do gutters.They do have an excellent reputation though, which is why I'm using them.)

Are they using 4" guttering?

AliceW said:

From an appearance point of view, if I have the shed where the red lines have been drawn, maybe I should think about having a longer shed -- I thought a longer  one might be more prone to leaking and more difficult to maintain? and I only need it for storing garden equipment and garden toys. A longer one would probably look better though.18ft instead of 16ft.Or maybe even 20ft.

Leaking shouldn't really be an issue, but many shed manufacturers construct longer sheds using two panels joined together in the middle. So for an 18' shed it may come with the long sides formed of two 9' panels.  That isn't really a problem in itself, but the longer the shed, the more the middle of the long sides can be prone to movement out of vertical.  (the long side is braced at the ends by the end panel, there's nothing in the middle to do the same).

This can be more of an issue with pent sheds - with apex sheds the load each side should be roughly equal, and there should be a tie to stop the sides spreading out.  But with a pent shed you don't normally have this.

Personally, for a long shed, I'd put in a braced stub-partition wall (say about 2' long) somewhere near the mid-point along the shed's length to give it more lateral stability.  It might be a bit inconvenient floorspace-wise, but would extend the life of the shed.  An alternative might having diagonal bracing on the underside of the rafters, but this is likely to be something to hit your head on.

subjecttocontract

AliceW said:

EssexExile said:

My wooden shed has been sitting on concrete for over 20 years with no problems.

As some have said it's a problem that the shed is smaller than the base, would it be possible to have the shed made to fully cover the base? A shed is never too big!

Thank you. I think I might go for a bigger one.

I can trump that. I have a summerhouse thats been sitting on concrete for 30 years this year. Two caveats:
* Concrete slab slightly smaller than summerhouse.
* Concrete slab laid with a damp proof membrane to stop rising damp.

Albermarle

ButterCheese said:

TELLIT01 said:

ButterCheese said:

I would also advise against a pre-fabricated shed as they don't use very thick wood for the supporting structures.  Perhaps buy the timber yourself and get someone to put it together if you can't.  We have asked our local roofer to build us a shed for this reason.

Lastly, I've seen a lots of landscapers use damp proof course or damp proof tape on shed bases as well.  Might be something to consider - I have no way of comparing them to a shed used without anything, but for a small outlay it's probably worth it.  My MIL has a small wooden shed which was put in 10 years ago, dorectly on top of paving slabs.  One side of the shed has now rotted at the bottom and is leaning

Claiming that pre-fabricated sheds don't use very thick wood is a massive generalisation.   

Just putting it out there, having watched several comparison vids on YouTube and also advice from our roofer who put up sheds full time for 6 years.  Yes there are proabbly good ones out there but the majority of people would buy one from a "sheds r us" type of website

When we had some garden work, fences installed, we needed a new shed. The guy/team doing the work said they could build one for about £900, which was a similar price to buying a very good quality one from a local company/garden centre at the time.
However from a MSE perspective it seemed a lot for a medium sized shed.
So I  bought one from a 'Sheds R Us' type company, but bought one of  their better quality ones for £450.( you could buy them at half that price) and it has been fine . I actually got the garden guys to put it up for me and they were quite complimentary about it.
So like many items, there are cheap and cheerful ones, expensive top of the range ones and those in the middle.